It is now well known to utilize a linearly-extended array of a plurality N of sensors to form a reception energy beam which can provide information as to the range and direction to an external source of the energy received by the array. Such arrays can be utilized with sound energy, particularly where the propagation medium is a liquid; in such sound-acquisition-and-ranging (sonar) arrays, typical acoustic frequencies in the 10-1000 Hz. range are used. The (sonar) array may be a linearly-extended array, extended along a straight line curve, which array is towed through the water by a ship. It is well known that, in order to achieve diffraction-limited or near-diffraction-limited beam-forming capability in towed linear sonar arrays, it is essential that any deviation from a linear common axis be known and that phase correction be made for each sensor element, by an amount which will vary accordingly to the deviation of that particular sensor from the common axis line. A monotonically-curved array will suffer a defocusing effect without such correction; an array undergoing a more complex form of curvature will also suffer from higher order aberrations.
Many approaches have been suggested for determining the shape of such an elongated array. In one example, magnetic heading sensors, based on compass action, have been suggested to provide information as to the angular orientation of each sensor element in the array; an assumed analytic curve can be fitted to match the angular heading data of each array sensor, if known. Another suggested approach is measurement of the translational velocity of each array element, with respect to the medium (water) through which the array is travelling, and the integration of this translational velocity data to provide element displacement; this method is based upon an assumption that displacements average out to zero and must provide a uniform starting constant for an integration process. Thus, some assumption is required in all of the hitherto-suggested approaches, e.g. an assumed curve description, use of a zero mean value, and the like. The resulting displacements are thus theoretically only approximations. It is highly desirable to provide apparatus for measuring actual displacements, with which data a method for correcting the phase of the signal from each of a plurality of array sensor elements can be provided to restore the array to diffraction-limited operation.